Compact discs (CDs) have conventionally been used as optical recording medium reproducing apparatus of this kind, wherein the disc as an optical recording medium is irradiated via an optical system 0.45 in numerical aperture (NA) with a laser beam 780 [nm] in wave length which is generated by a laser diode.
In recent years, however, new laser beam sources with a wave length smaller than 780 [nm] (for example, red=680 [nm], and semiconductor lasers of green and blue) have been developed as light sources for optical recording medium reproducing apparatuses. These new laser beam sources serve to implement recording medium reproducing apparatuses that can reproduce recording media with a higher recording density than compact discs. These recording medium reproducing apparatuses that can reproduce recording media with a higher recording density than compact discs desirably have a compatible reproducing function that also enables conventional compact discs (CDs) to be reproduced.
The diameter of the light spot that can be formed by a laser beam of a large wave length is selected so as to be somewhat larger than the width W1 of pits P1 formed in the compact disc as recorded information as shown by reference L1 in FIG. 1(A). This allows the light spot L1 to constantly lie across the width W1 during movement when entering the pit P1 from land to scan it.
A sum signal (hereafter referred to as an "RF signal") can thus be obtained from an optical pickup based on light reflected from the compact disc because of the light spot L1 in FIG. 1(A). The RF signal falls from a first signal level LV11 to a second signal level LV12 when the light spot L1 passes through the end of the pit P1, and subsequently maintains the signal level LV12 until the light spot L1 has passed the pit P1 with itself lying across the pit P1, as shown in FIG. 1(B). This results in a sum signal in which the signal level changes in response to the lengths of the land and the pit P1 because the light spot L1 scans both the land and the pit P1.
Thus, the signal level of the RF signal RF1 decreases during scanning due to the interference between light reflected from the pit P1 and light reflected from a reflecting surface (the land) located around the pit P1. This also occurs when a light spot L2 of a relatively small wave length scans a pit P2 formed in an optical disc that has a higher recording density than the compact discs.
Each time the light spot L2 formed by a laser beam of a relatively small wave length enters the pit P2, an RF signal RF2, the signal level of which changes from LV21 to LV22 in response to recorded information can be obtained as shown in FIG. 2(B).
The light spot L2 formed by a laser beam of a relatively small wave length can be converged on a small diameter compared to the light spot L1 formed by a laser beam of a relatively large wave length, so the width W2 of the pit P2 may be smaller than the width W1 of the pit P1. As a result, optical recording medium reproducing apparatuses using the light spot L2 formed by a laser beam of a relatively small wave length deal with a high recording density, while optical recording medium reproducing apparatuses using the light spot L1 formed by a laser beam of a relatively large wave length cope with a low recording density.
If a high recording density optical recording medium reproducing apparatus is used to directly reproduce a compact disc (CD) designed to be reproduced by a low recording density optical recording medium reproducing apparatus, the diameter of the light spot L2 is smaller than or equal to the width W1 of the pit P1, as shown in FIG. 3(A). In this case, light reflected from the land and light reflected from the pit P1 interfere with each other when the light spot L2 enters and leaves the pit P1, whereas no interference occurs while the light spot L2 is scanning the pit P1 because it is totally included within the pit P1. The signal level of an RF signal RF3 changes from LV31 to LV32 only at both ends of the pit P1, as shown in FIG. 3(B).
Although the signals shown in FIGS. 1(B) and 2(B) can be detected by integral detection, the signal shown in FIG. 3(B) cannot be detected by such detection but requires differential detection. The differential detection, however, has a higher error rate than the integral detection.
This invention is proposed in view of the above points, and its objective is to provide an optical recording medium reproducing apparatus that performs reproducing operations using a light spot formed by a laser beam of a relatively small wave length and which can compatibly reproduce low recording density optical recording media.